Bug: setDefaultDimensions() ignores regular dimensions in limit check

[svozza]

Expected Behavior

setDefaultDimensions() should account for all dimensions (default + regular + dimension sets) when enforcing the maximum dimension count, ensuring the total never exceeds CloudWatch's limit.

Current Behavior

setDefaultDimensions() at Metrics.ts:796-813 only checks the count of existing default dimensions against the limit, completely ignoring regular dimensions added via addDimension():

const currentCount = Object.keys(currentDefaultDimensions).length;
const newSetCount = Object.keys(newDimensions).length;
if (currentCount + newSetCount >= MAX_DIMENSION_COUNT) {
  throw new RangeError(...);
}

You can exceed CloudWatch's limit by adding 25 regular dimensions, then calling setDefaultDimensions() with 20 new keys — the check only sees 1 (service) + 20 = 21 < 29 and allows it, even though the total is now 46.

CloudWatch will silently drop the metric.

Code snippet

import { Metrics } from '@aws-lambda-powertools/metrics';

const metrics = new Metrics({ namespace: 'test' });

// Add 25 regular dimensions (plus 'service' default = 26 total)
for (let i = 0; i < 25; i++) {
  metrics.addDimension(`regular-${i}`, 'test');
}

// ❌ Should throw but doesn't — only checks default count (1) + new (20) = 21 < 29
const newDefaults: Record<string, string> = {};
for (let i = 0; i < 20; i++) {
  newDefaults[`new-default-${i}`] = 'test';
}
metrics.setDefaultDimensions(newDefaults);
// No error — but total dimensions is now 46, well above CloudWatch's limit

Steps to Reproduce

1. Create a Metrics instance (1 default service dimension)
2. Add 25 regular dimensions via addDimension() (total = 26)
3. Call setDefaultDimensions() with 20 new keys
4. Observe no error, even though total dimensions = 46

Possible Solution

Count only the genuinely new keys that don't overlap with existing defaults, then check against the full dimension count:

const newKeysCount = Object.keys(newDimensions)
  .filter((key) => !Object.hasOwn(currentDefaultDimensions, key)).length;
const totalAfterMerge = this.#dimensionsStore.getDimensionCount() + newKeysCount;
if (totalAfterMerge >= MAX_DIMENSION_COUNT) {
  throw new RangeError(...);
}

Powertools for AWS Lambda (TypeScript) version

2.33.0

AWS Lambda function runtime

22.x

Packaging format used

npm

---

Disclaimer: After creating an issue, please wait until it is triaged and confirmed by a maintainer before implementing it. This will reduce amount of rework and the chance that a pull request gets rejected.